1. Field of Invention
The present invention relates generally to holographic laser scanners of ultra-compact design capable of reading bar and other types of graphical indicia within a large scanning volume using holographic optical elements and visible laser diodes, and also a method of designing and operating the same for use in diverse applications.
2. Brief Description of the Prior Art
The use of bar code symbols for product and article identification is well known in the art. Presently, various types of bar code symbol scanners have been developed. In general, these bar code symbol readers can be classified into two distinct groups.
The first class of bar code symbol reader simultaneously illuminates all of the bars and spaces of a bar code symbol with light of a specific wavelength(s) in order to capture an image thereof for recognition/decoding purposes. Such scanners are commonly known as CCD scanners because they use CCD image detectors to detect images of the bar code symbols being read.
The second class of bar code symbol reader uses a focused light beam, typically a focused laser beam, to sequentially scan the bars and spaces of a bar code symbol to be read. This type of bar code symbol scanner is commonly called a "flying spot" scanner as the focused laser beam appears as "a spot of light that flies" across the bar code symbol being read. In general, laser bar code symbol scanners are subclassified further by the type of mechanism used to focus and scan the laser beam across bar code symbols.
The majority of laser scanners in use today employ lenses and moving (i.e. rotating or oscillating) mirrors in order to focus and scan laser beams across bar code symbols during code symbol reading operations. Examples of such laser scanners are disclosed in great detail in the Background of Invention of U.S. Pat. Nos. 5,216,232 to Knowles et al.; 5,340,973 to Knowles et al.; 5,340,971 to Rockstein et al.; 5,424,525 to Rockstein et al., which are incorporated herein by reference.
One type of laser scanner that has enjoyed great popularity in recent years is called the "polygon scanner" in that it employs a rotating polygon whose sides bear light reflective surfaces (e.g. mirrors) for scanning a laser beam over multiple paths through space above the scanning window of the scanner. In polygon-type laser scanners, the angular sweep of the outgoing laser beam and the light collection efficiency of the return laser beam are both directly related to the number and size of light reflective facets on the rotating polygon.
In contrast to laser scanners, which use lenses (i.e. light refractive elements) to shape and focus laser light beams and light reflective surfaces to scan focused laser beams, there exists another subclass of laser scanner which employs a high-speed holographic disc. In general, the holographic disc comprises an array of holographic optical elements (HOEs) called "facets" which function to focus and deflect outgoing laser beams during laser beam scanning operations, as well as focus incoming reflected laser light during light collection/detection operations. Such bar code symbol scanners are typically called holographic laser scanners or readers because holographic optical elements (HOEs) are employed. Examples of prior art holographic scanners are disclosed in U.S. Pat. Nos. 4,415,224; 4,758,058; 4,748,316; 4,591,242; 4,548,463; 5,331,445 and 5,416,505, incorporated herein by reference.
Holographic laser scanners, or readers, have many advantages over laser scanners which employ lenses and mirrors for laser beam focusing and scanning (i.e. deflection) functions.
One of the major advantages of holographic laser scanners over polygon laser scanners is the ability of holographic laser scanners to independently control (i) the angular sweep of the outgoing laser beam and (ii) the light collection efficiency for the returning laser beam.
Holographic laser scanners have other advantages over polygon-type laser scanners. In particular, in holographic laser scanners, light collection efficiency is determined by the size of the light collecting portion of each holographic facet, while the angular sweep of the outgoing laser beam is determined by the angular width of the outgoing beam portion of the holographic facet and the angles of incidence and diffraction of the outgoing laser beam.
While prior art holographic scanning systems have many advantages over mirror-based laser scanning systems, prior art holographic scanners are not without problems.
In the first holographic scanner produced by International Business Machines (IBM), the holographic facets on its holographic disc were simple sectors which did not allow for independent control over light collection and light scanning functions. Consequently, such holographic scanners had faster scanning speeds than were needed for the applications at hand. Subsequent industrial scanners designed by IBM allowed independent control of these functions. However, the holographic discs employed in prior art holographic scanners, e.g. the HOLOSCAN 2100.TM. holographic laser scanner designed and sold by Holoscan, Inc. of San Jose, Calif., fail to (i) maximize the use of available space on the disc for light collection purposes, and (ii) minimize the scan line speed for particular laser scanning patterns. As a result of such design limitations, prior art holographic scanners have required the use of large scanning discs which make inefficient use of the available light collecting surface area thereof. They also are incapable of producing from each holographic facet thereon, detected scan data signals having substantially the same signal level independent of the location in the scanning volume from which the corresponding optical scan data signal is produced. Consequently, this has placed great demands on the electrical signal processing circuitry required to handle the dramatic signal swings associated with such detected return signals.
While U.S. Pat. No. 4,415,224 to Applicant (Dickson) discloses a method of equalizing the light collection efficiency of each facet on the holographic scanning disc, it does not disclose, teach or suggest a method of equalizing the light collection efficiency of each facet on the holographic scanning disc, while utilizing substantially all of the light collecting surface area thereof. Thus, in general, prior art holographic laser scanners have required very large scanner housings in order to accommodate very large scanning discs using only a portion of their available light collection surface area.
In many code symbol reading applications, the volumetric extent of the holographic scanner housing must be sufficiently compact to accommodate the small volume of space provided for physical installation. However, due to limitations of conventional design principles, it has not been possible to build prior art holographic scanners having sufficient compactness required in many applications. Consequently, the huge housings required to enclose the optical apparatus of prior art holographic laser scanners have restricted their use to only a few practical applications where housing size constraints are of little concern.
While highly desirable because of their low power usage and miniature size, solid-state visible laser diodes (VLDs) cannot be used practically in prior art holographic laser scanners because of several problems which arise from inherent properties of conventional VLDs.
The first problem associated with the use of VLDs in holographic laser scanners is that the VLDs do not produce a single spectral line output in the manner of conventional He-Ne laser tubes. Rather, conventional VLDs always produce some background super-luminescence, which is a broad spectrum of radiation of the type produced by conventional light emitting diodes (LEDs). Also, VLDs often operate in more than one oscillation mode and/or exhibit mode hopping, in which the VLD jumps from one mode of oscillation to another. Both of these characteristics of VLDs result in a spreading of the laser beam as it leaves the highly dispersive holographic facet of the holographic disc. This results in an effectively larger "spot" at the focal point of the holographic facet, causing errors in the resolution of the bars and spaces of scanned code symbols and, often, intolerable symbol decoding errors.
The second problem associated with the use of VLDs in a holographic scanner is that the inherent "astigmatic difference" in VLDs results in the production of laser beams exhibiting astigmatism along the horizontal and vertical directions of propagation. This fact results in the outgoing laser beam having a cross-sectional dimension whose size and orientation varies as a function of distance away from the VLD. Thus, at particular points in the scanning field of a holographic scanner using a VLD, the orientation of the laser beam ("flying spot") will be such that the bars and spaces cannot be resolved for symbol decoding operations.
Holographic scanners suffer from other technical problems as well.
In prior art holographic scanners, the light collection and detection optics are necessarily complicated and require a significant volume of space within the scanner housing. This necessarily causes the height dimension of the scanner housing to be significantly larger than desired in nearly all code symbol reading applications.
When an outgoing laser beam passes though, and is diffracted by, the rotating holographic facets of prior art holographic scanners, "holographically-introduced" astigmatism is inherently imparted to the outgoing laser beam. While the source of this type of astigmatism is different than the source of astigmatism imparted to a laser beam due to the inherent astigmatic difference in VLDs, the effect is substantially the same, namely: the outgoing laser beam has a cross-sectional dimension whose size and orientation varies as a function of distance away from the holographic facet. Thus, at particular points in the scanning field of a holographic scanner, the orientation of the laser beam (i.e. "the flying spot") will be such that the bars and spaces of a scanned bar code symbol cannot be resolved for symbol decoding operations. Consequently, it has been virtually impossible to design a holographic laser scanner with a three-dimensional scanning volume that is capable of scanning bar code symbols independent of their orientation as they move through the scanning volume.
Because of the methods used to design and construct prior art holographic disks, the size and shape of the light collection area of each facet could not be controlled independent of the angular sweep of the outgoing laser beam. Consequently, this has prevented optimal use of the disk surface area for light collection functions, and thus the performance of prior art holographic scanners has been necessarily compromised.
While the above problems generally define the major areas in which significant improvement is required of prior art holographic laser scanners, there are still other problems which have operated to degrade the performance of such laser scanning systems.
In particular, glare produced by specular reflection of a laser beam scanning a code symbol reduces the detectable contrast of the bars and spaces of the symbol against its background and thus the SNR of the optical scan data signal detected at the photodetectors of the system. While polarization filtering techniques are generally known for addressing such problems in laser scanning systems, it is not known how such techniques might be successfully applied to holographic type laser scanning systems while simultaneously solving the above-described problems.
Thus, there is a great need in the art for an improved holographic laser scanning system and a method of designing and constructing the same, while avoiding the shortcomings and drawbacks of prior art holographic scanners and methodologies.